tion between a growing population and fixed natural resources must lead to diminishing returns per person, other things being equal. Even if we allow for the fact that capital is, in some sense and to some degree, a substitute for natural resourcesgiven the belief that capital becomes a successively less adequate substitute as more capital is substituted for resourcesdiminishing returns to population growth appear to be valid and reasonable. In principle, output per worker must eventually decline as the population grows. The one ameliorating influence is the application of new inventions to the production process. At first blush, this view seems so reasonable that it is difficult to believe that it could be faulted. Probably most reasoned arguments about the hazards of population growth to economic welfare depend in one way or another on arguments of this type. Such arguments appear to have special relevance for developing countries where the bulk of the output takes place in the agricultural sector and where it often appears that the genuine limiting factor must be cultivable land. In what follows it will not be argued that these traditional considerations are of no importance. Far from it. Rather, the thrust of the argument is that such considerations are less important than one would have believed 2 decades ago and that in most instances they are not the factors of prime importance. The endless stream of arguments about the applicability of neo-Malthusian models rests on two major elements: 1. The consequences of technical change resulting from an endless flow of inventions enable us to avoid indefinitely an approximation to the state of Malthusian equilibrium. 2. The fertility assumptions employed in the neo-Malthusian models no longer hold in view of contemporary contraceptive technology. Although the second objection is probably a weak one,* the first one seemed to be strong and to have considerable basis in fact. As a consequence, the neo-Malthusian debate frequently turned on whether one happened to be an invention-innovation optimist or an invention-innovation pessimist; i.e., whether one believed that the current rate of material inventions would continue or one believed that this, too, was subject to considerable diminishing returns. The experience of the last half century or so supports the technological optimists. Extrapolating from countries and periods within the last 50 years, during which fairly rapid growth has taken place, it would appear that at least potential economic growth is greater than the rate of population growth. The argument of the technological pessimists depends on theory rather than experience, i.e., on the belief that some resources such as land are in fact fixed and that, in fact, the substitutes for such resources are likely to be considerably inferior to the fixed resources so that diminishing returns are likely to result.